Encryption Library

The Encryption Library provides two-way data encryption. To do so in a cryptographically secure way, it utilizes PHP extensions that are unfortunately not always available on all systems. You must meet one of the following dependencies in order to use this library:

• OpenSSL
• MCrypt (and MCRYPT_DEV_URANDOM availability)

If neither of the above dependencies is met, we simply cannot offer you a good enough implementation to meet the high standards required for proper cryptography.

• Using the Encryption Library
  • Initializing the Class
  • Default behavior
  • Setting your encryption_key
  • Supported encryption ciphers and modes
    • Portable ciphers
    • Driver-specific ciphers
    • Encryption modes
  • Message Length
  • Configuring the library
  • Encrypting and decrypting data
    • How it works
    • Using custom parameters
    • Supported HMAC authentication algorithms
• Class Reference

Using the Encryption Library

Initializing the Class

Like most other classes in CodeIgniter, the Encryption library is initialized in your controller using the $this->load->library() method:

$this->load->library('encryption');

Once loaded, the Encryption library object will be available using:

$this->encryption

Default behavior

By default, the Encryption Library will use the AES-128 cipher in CBC mode, using your configured encryption_key and SHA512 HMAC authentication.

However, the encryption_key is not used as is.

If you are somewhat familiar with cryptography, you should already know that a HMAC also requires a secret key and using the same key for both encryption and authentication is a bad practice.

Because of that, two separate keys are derived from your already configured encryption_key: one for encryption and one for authentication. This is done via a technique called HMAC-based Key Derivation Function (HKDF).

Setting your encryption_key

An encryption key is a piece of information that controls the cryptographic process and permits a plain-text string to be encrypted, and afterwards - decrypted. It is the secret “ingredient” in the whole process that allows you to be the only one who is able to decrypt data that you’ve decided to hide from the eyes of the public. After one key is used to encrypt data, that same key provides the only means to decrypt it, so not only must you chose one carefully, but you must not lose it or you will also lose access to the data.

It must be noted that to ensure maximum security, such key should not only be as strong as possible, but also often changed. Such behavior however is rarely practical or possible to implement, and that is why CodeIgniter gives you the ability to configure a single key that is to be used (almost) every time.

It goes without saying that you should guard your key carefully. Should someone gain access to your key, the data will be easily decrypted. If your server is not totally under your control it’s impossible to ensure key security so you may want to think carefully before using it for anything that requires high security, like storing credit card numbers.

Your encryption key must be as long as the encyption algorithm in use allows. For AES-128, that’s 128 bits or 16 bytes (charcters) long. You will find a table below that shows the supported key lengths of different ciphers.

The key should be as random as possible and it must not be a regular text string, nor the output of a hashing function, etc. In order to create a proper key, you must use the Encryption library’s create_key() method

// $key will be assigned a 16-byte (128-bit) random key
$key = $this->encryption->create_key(16);

The key can be either stored in your application/config/config.php, or you can design your own storage mechanism and pass the key dynamically when encrypting/decrypting.

To save your key to your application/config/config.php, open the file and set:

$config['encryption_key'] = 'YOUR KEY';

You’ll notice that the create_key() method outputs binary data, which is hard to deal with (i.e. a copy-paste may damage it), so you may use bin2hex(), hex2bin() or Base64-encoding to work with the key in a more friendly manner. For example:

// Get a hex-encoded representation of the key:
$key = bin2hex($this->encryption->create_key(16));

// Put the same value in your config with hex2bin(),
// so that it is still passed as binary to the library:
$config['encryption_key'] = hex2bin(<your hex-encoded key>);

Supported encryption ciphers and modes

Portable ciphers

Because MCrypt and OpenSSL (also called drivers throughout this document) each support different sets of encryption algorithms and often implement them in different ways, our Encryption library is designed to use them in a portable fashion, or in other words - it enables you to use them interchangeably, at least for the ciphers supported by both drivers.

It is also implemented in a way that aims to match the standard implementations in other programming languages and libraries.

Here’s a list of the so called “portable” ciphers, where “CodeIgniter name” is the string value that you’d have to pass to the Encryption library to use that cipher:

Cipher name	CodeIgniter name	Key lengths (bits / bytes)	Supported modes
AES-128 / Rijndael-128	aes-128	128 / 16	CBC, CTR, CFB, CFB8, OFB, ECB
AES-192	aes-192	192 / 24	CBC, CTR, CFB, CFB8, OFB, ECB
AES-256	aes-256	256 / 32	CBC, CTR, CFB, CFB8, OFB, ECB
DES	des	56 / 7	CBC, CFB, CFB8, OFB, ECB
TripleDES	tripledes	56 / 7, 112 / 14, 168 / 21	CBC, CFB, CFB8, OFB
Blowfish	blowfish	128-448 / 16-56	CBC, CFB, OFB, ECB
CAST5 / CAST-128	cast5	88-128 / 11-16	CBC, CFB, OFB, ECB
RC4 / ARCFour	rc4	40-2048 / 5-256	Stream

Driver-specific ciphers

As noted above, MCrypt and OpenSSL support different sets of encryption ciphers. For portability reasons and because we haven’t tested them properly, we do not advise you to use the ones that are driver-specific, but regardless, here’s a list of most of them:

Cipher name	Driver	Key lengths (bits / bytes)	Supported modes
AES-128	OpenSSL	128 / 16	CBC, CTR, CFB, CFB8, OFB, ECB, XTS
AES-192	OpenSSL	192 / 24	CBC, CTR, CFB, CFB8, OFB, ECB, XTS
AES-256	OpenSSL	256 / 32	CBC, CTR, CFB, CFB8, OFB, ECB, XTS
Rijndael-128	MCrypt	128 / 16, 192 / 24, 256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
Rijndael-192	MCrypt	128 / 16, 192 / 24, 256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
Rijndael-256	MCrypt	128 / 16, 192 / 24, 256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
GOST	MCrypt	256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
Twofish	MCrypt	128 / 16, 192 / 24, 256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
CAST-128	MCrypt	40-128 / 5-16	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
CAST-256	MCrypt	128 / 16, 192 / 24, 256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
Loki97	MCrypt	128 / 16, 192 / 24, 256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
SaferPlus	MCrypt	128 / 16, 192 / 24, 256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
Serpent	MCrypt	128 / 16, 192 / 24, 256 / 32	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
XTEA	MCrypt	128 / 16	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
RC2	MCrypt	8-1024 / 1-128	CBC, CTR, CFB, CFB8, OFB, OFB8, ECB
RC2	OpenSSL	8-1024 / 1-128	CBC, CFB, OFB, ECB
Camellia-128	OpenSSL	128 / 16	CBC, CFB, CFB8, OFB, ECB
Camellia-192	OpenSSL	192 / 24	CBC, CFB, CFB8, OFB, ECB
Camellia-256	OpenSSL	256 / 32	CBC, CFB, CFB8, OFB, ECB
Seed	OpenSSL	128 / 16	CBC, CFB, OFB, ECB

Encryption modes

Different modes of encryption have different characteristics and serve for different purposes. Some are stronger than others, some are faster and some offer extra features. We are not going in depth into that here, we’ll leave that to the cryptography experts. The table below is to provide brief informational reference to our more experienced users. If you are a beginner, just stick to the CBC mode - it is widely accepted as strong and secure for general purposes.

Mode name	CodeIgniter name	Driver support	Additional info
CBC	cbc	MCrypt, OpenSSL	A safe default choice
CTR	ctr	MCrypt, OpenSSL	Considered as theoretically better than CBC, but not as widely available
CFB	cfb	MCrypt, OpenSSL	N/A
CFB8	cfb8	MCrypt, OpenSSL	Same as CFB, but operates in 8-bit mode (not recommended).
OFB	ofb	MCrypt, OpenSSL	N/A
OFB8	ofb8	MCrypt	Same as OFB, but operates in 8-bit mode (not recommended).
ECB	ecb	MCrypt, OpenSSL	Ignores IV (not recommended).
XTS	xts	OpenSSL	Usually used for encrypting random access data such as RAM or hard-disk storage.
Stream	stream	MCrypt, OpenSSL	This is not actually a mode, it just says that a stream cipher is being used. Required because of the general cipher+mode initialization process.

Message Length

It’s probably important for you to know that an encrypted string is usually longer than the original, plain-text string (depending on the cipher).

This is influenced by the cipher algorithm itself, the IV prepended to the cipher-text and the HMAC authentication message that is also prepended. Furthermore, the encrypted message is also Base64-encoded so that it is safe for storage and transmission, regardless of a possible character set in use.

Keep this information in mind when selecting your data storage mechanism. Cookies, for example, can only hold 4K of information.

Configuring the library

For usability, performance, but also historical reasons tied to our old Encrypt Class, the Encryption library is designed to use repeatedly the same driver, encryption cipher, mode and key.

As noted in the “Default behavior” section above, this means using an auto-detected driver (OpenSSL has a higher priority), the AES-128 ciper in CBC mode, and your $config['encryption_key'] value.

If you wish to change that however, you need to use the initialize() method. It accepts an associative array of parameters, all of which are optional:

Option	Possible values
driver	‘mcrypt’, ‘openssl’
cipher	Cipher name (see Supported encryption ciphers and modes)
mode	Encryption mode (see Encryption modes)
key	Encryption key

For example, if you were to change the encryption algorithm and mode to AES-256 in CTR mode, this is what you should do:

$this->encryption->initialize(
        array(
                'cipher' => 'aes-256',
                'mode' => 'ctr',
                'key' => '<a 32-character random string>'
        )
);

Note that we only mentioned that you want to change the ciper and mode, but we also included a key in the example. As previously noted, it is important that you choose a key with a proper size for the used algorithm.

There’s also the ability to change the driver, if for some reason you have both, but want to use MCrypt instead of OpenSSL:

// Switch to the MCrypt driver
$this->encryption->initialize(array('driver' => 'mcrypt'));

// Switch back to the OpenSSL driver
$this->encryption->initialize(array('driver' => 'openssl'));

Encrypting and decrypting data

Encrypting and decrypting data with the already configured library settings is simple. As simple as just passing the string to the encrypt() and/or decrypt() methods:

$plain_text = 'This is a plain-text message!';
$ciphertext = $this->encryption->encrypt($plain_text);

// Outputs: This is a plain-text message!
echo $this->encryption->decrypt($ciphertext);

And that’s it! The Encryption library will do everything necessary for the whole process to be cryptographically secure out-of-the-box. You don’t need to worry about it.

How it works

If you must know how the process works, here’s what happens under the hood:

• $this->encryption->encrypt($plain_text)
  1. Derive an encryption key and a HMAC key from your configured encryption_key via HKDF, using the SHA-512 digest algorithm.
  2. Generate a random initialization vector (IV).
  3. Encrypt the data via AES-128 in CBC mode (or another previously configured cipher and mode), using the above-mentioned derived encryption key and IV.
  4. Prepend said IV to the resulting cipher-text.
  5. Base64-encode the resulting string, so that it can be safely stored or transferred without worrying about character sets.
  6. Create a SHA-512 HMAC authentication message using the derived HMAC key to ensure data integrity and prepend it to the Base64 string.
• $this->encryption->decrypt($ciphertext)
  1. Derive an encryption key and a HMAC key from your configured encryption_key via HKDF, using the SHA-512 digest algorithm. Because your configured encryption_key is the same, this will produce the same result as in the encrypt() method above - otherwise you won’t be able to decrypt it.
  2. Check if the string is long enough, separate the HMAC out of it and validate if it is correct (this is done in a way that prevents timing attacks against it). Return FALSE if either of the checks fails.
  3. Base64-decode the string.
  4. Separate the IV out of the cipher-text and decrypt the said cipher-text using that IV and the derived encryption key.

Using custom parameters

Let’s say you have to interact with another system that is out of your control and uses another method to encrypt data. A method that will most certainly not match the above-described sequence and probably not use all of the steps either.

The Encryption library allows you to change how its encryption and decryption processes work, so that you can easily tailor a custom solution for such situations.

All you have to do is to pass an associative array with a few parameters to either the encrypt() or decrypt() method. Here’s an example:

// Assume that we have $ciphertext, $key and $hmac_key
// from on outside source

$message = $this->encryption->decrypt(
        $ciphertext,
        array(
                'cipher' => 'blowfish',
                'mode' => 'cbc',
                'key' => $key,
                'hmac_digest' => 'sha256',
                'hmac_key' => $hmac_key
        )
);

In the above example, we are decrypting a message that was encrypted using the Blowfish cipher in CBC mode and authenticated via a SHA-256 HMAC.

Below is a list of the available options.

However, unless you really need to and you know what you are doing, we advise you to not change the encryption process as this could impact security, so please do so with caution.

Option	Default value	Mandatory / Optional	Description
cipher	N/A	Yes	Encryption algorithm (see Supported encryption ciphers and modes).
mode	N/A	Yes	Encryption mode (see Encryption modes).
key	N/A	Yes	Encryption key.
hmac	TRUE	No	Whether to use a HMAC. Boolean. If set to FALSE, then hmac_digest and hmac_key will be ignored.
hmac_digest	sha512	No	HMAC message digest algorithm (see Supported HMAC authentication algorithms).
hmac_key	N/A	Yes, unless hmac is FALSE	HMAC key.
raw_data	FALSE	No	Whether the cipher-text should be raw. Boolean. If set to TRUE, then Base64 encoding and decoding will not be performed and HMAC will not be a hexadecimal string.

Supported HMAC authentication algorithms

For HMAC message authentication, the Encryption library supports usage of the SHA-2 family of algorithms:

Algorithm	Raw length (bytes)	Hex-encoded length (bytes)
sha512	64	128
sha384	48	96
sha256	32	64
sha224	28	56

The reason for not including other popular algorithms, such as MD5 or SHA1 is that they are no longer considered secure enough and as such, we don’t want to encourage their usage. If you absolutely need to use them, it is easy to do so via PHP’s native hash_hmac() function.

Stronger algorithms of course will be added in the future as they appear and become widely available.

Class Reference

class CI_Encryption

initialize($params)

Parameters:	$params (array) – Configuration parameters
Returns:	CI_Encryption instance (method chaining)
Return type:	CI_Encryption

Initializes (configures) the library to use a different driver, cipher, mode or key.

Example:

$this->encryption->initialize(
        array('mode' => 'ctr')
);

Please refer to the Configuring the library section for detailed info.

encrypt($data[, $params = NULL])

Parameters:	$data (string) – Data to encrypt $params (array) – Optional parameters
Returns:	Encrypted data or FALSE on failure
Return type:	string

Encrypts the input data and returns its ciphertext.

Example:

$ciphertext = $this->encryption->encrypt('My secret message');

Please refer to the Using custom parameters section for information on the optional parameters.

decrypt($data[, $params = NULL])

Parameters:	$data (string) – Data to decrypt $params (array) – Optional parameters
Returns:	Decrypted data or FALSE on failure
Return type:	string

Decrypts the input data and returns it in plain-text.

Example:

echo $this->encryption->decrypt($ciphertext);

Please refer to the Using custom parameters secrion for information on the optional parameters.

create_key($length)

Parameters:	$length (int) – Output length
Returns:	A pseudo-random cryptographic key with the specified length, or FALSE on failure
Return type:	string

Creates a cryptographic key by fetching random data from the operating system’s sources (i.e. /dev/urandom).

hkdf($key[, $digest = 'sha512'[, $salt = NULL[, $length = NULL[, $info = '']]]])

Parameters:	$key (string) – Input key material $digest (string) – A SHA-2 family digest algorithm $salt (string) – Optional salt $length (int) – Optional output length $info (string) – Optional context/application-specific info
Returns:	A pseudo-random key or FALSE on failure
Return type:	string

Derives a key from another, presumably weaker key.

This method is used internally to derive an encryption and HMAC key from your configured encryption_key.

It is publicly available due to its otherwise general purpose. It is described in RFC 5869.

However, as opposed to the description in RFC 5869, this implementation doesn’t support SHA1.

Example:

$hmac_key = $this->encryption->hkdf(
        $key,
        'sha512',
        NULL,
        NULL,
        'authentication'
);

// $hmac_key is a pseudo-random key with a length of 64 bytes